In the anatomy of the spine, the sacrum, situated beneath the lumbar vertebrae, is constituted by five vertebrae which, over the course of human evolution, have become fused together. The top vertebra of the sacrum, written S1, is articulated to the fifth lumbar vertebra, written L5. This articulated connection constitutes the lumbo-sacral joint, or the L5-S1 joint, and is shown in FIG. 1.
Each lumbar vertebra presents a middle and posterior projection: the spinous process, sometimes referred to below as the process 10. The sacral vertebrae have lost their spinous processes during evolution, and instead they retain a small residual bulge 12.
In man, certain kinds of back pain can be due to stresses associated with relative movements between two vertebrae acting on the intervertebral disk situated between those vertebrae.
Numerous intervertebral implants are already known that seek to limit the movements of two vertebrae relative to each other so as to relieve the intervertebral disk, and in particular the implant described in document FR 2 775 183. That implant is a spacer presenting two longitudinal notches on its top and bottom faces extending in the same direction, that of the midplane of the spacer, for the purpose of receiving the spinous processes of the adjacent vertebrae between which the spacer is to be implanted. The spacer is then held in position by straps surrounding said processes. By blocking a portion of the spine, the spacer transfers loads from above and below the vertebrae concerned, thereby relieving the intervertebral disk situated between these vertebrae. Unfortunately, because of the anatomy of the sacral region, and more particularly because of the lack of a spinous process on vertebra S1, it is not possible to put that type of spacer into place over the L5-S1 joint.
A type of implant described in document EP 1 13 8 2 68 is also known that is specifically adapted for the anatomy of the lumbo-sacral region. That implant comprises an intervertebral spacer and a link bar. The intervertebral spacer presents two notches that are substantially mutually orthogonal, and the link bar is secured to the sacrum by means of two hooks secured to the vertebra S1. More precisely, the hooks bear against the top portion of the vertebra S1, also known as the posterior arc, and each of them is secured to the sacrum by fastener means such as staples that enable the hooks to be put into position and stabilized. Once the hooks are installed on the sacrum, the bar is secured to the hooks and the intervertebral spacer is put into place. The top notch in the spacer is suitable for receiving the spinous process on vertebra L5, while the bottom notch is of a shape that is suitable for receiving the bar, such that the spacer rests on the bar.
Nevertheless, that type of implant suffers from numerous drawbacks. Firstly, in the short term, fastening hooks to the sacrum, e.g. by staples, can be traumatic for the patient. Subsequently, in the medium to long term, the compression and extension stresses to which the implant is subjected are transferred to the means for fastening the hooks to the sacrum and lead to the holes in which the fastener means are secured becoming larger. Play is then created between the sacrum and the hooks, which can lead to the implant having poor mechanical behavior, or even to the fastener means being torn loose. The trauma suffered by the patient is then major, and a new operation must be envisaged in order to withdraw and possibly replace the defective implant.
The present invention seeks to solve the drawbacks of existing devices.